Blizzard has sent an in-game mail to everyone explaining what went wrong with the mail system, and what will be happening to fix it.
In other news, the world blew up from all the irony. The mice have quickly begun repairing the world, and hope to have a new one constructed shortly.
Basically the mail is outlining what Zarhym told us all earlier: that they will be reimbursing the items and gold lost during today's in-game mail failure, or mailure as I've started to call it tonight.
Full text of the in-game mail after the break, or you can just look at a large screenshot of the message.